1. Field of the Invention
The present invention relates to an objective lens, an optical pickup and, an optical disc apparatus, and can be appropriately applied to, for example, an optical disc apparatus that records information on an optical disc and reproduces information from the corresponding optical disc.
2. Description of the Related Art
Generally, optical disc apparatuses which record information on an optical disc such as a CD (Compact Disc) and a DVD (Digital Versatile Disc) and read the corresponding information from the corresponding optical disc have been widely used. In recent years, additionally, a Blu-ray Disc (registered trademark, hereinafter referred to as BD) in which a density of recording information on an optical disc is significantly increased has come into wide spread use.
The optical disc apparatuses are configured to focus a light beam onto a track spirally or concentrically formed on a recording layer of an optical disc through an objective lens and keep track of the focal point.
For example, in the case of the DVD format, the objective lens focuses a light beam with a wavelength of about 660 nm onto the recording layer which is formed below a cover layer having a thickness of about 0.6 mm in the optical disc at a numerical aperture (NA) of about 0.6 or more.
In contrast, in the case of the BD format, in order to minimize a diameter of a light spot on the information recording surface of the optical disc, it is necessary for the objective lens to focus a light beam with a wavelength of about 405 nm onto the recording layer which is formed below the cover layer having a thickness of about 0.1 mm in the optical disc at a numerical aperture (NA) of 0.8 or more.
In recent years, a resin material highly resistant to blue laser light has been developed. Thus, from the viewpoint of reduction in cost, weight, or the like, an objective lens made of the resin material instead of the typical glass material will be introduced even into the BD format.
However, the refractive index of the objective lens made of the resin material is greatly dependent on temperature in comparison with that of the objective lens made of the glass material, and thus the optical characteristics thereof tend to change in accordance with temperature changes. On the other hand, in view of a usage environment, inner temperature rise, or the like, a guaranteed operation temperature range of the objective lens mounted in the optical disc apparatus is supposed to be, for example, a range of 0 to 70° C.
For example, in the optical disc apparatus using the objective lens made of the resin material, at a reference temperature set to 35° C. which is at substantially the middle of the operable temperature range, a 3rd-order spherical aberration amount was designed and adjusted to be substantially equal to 0 mλrms. In the optical disc apparatus, as indicated by the characteristic curve QTS in FIG. 1, the 3rd-order spherical aberration exceeds 70 mλrms of the Marechal criterion on both the high temperature side and the low temperature side of the range of 0 to 70° C., and thus there is a concern that a problem may arise in signal quality.
In the optical disc apparatus, there has been proposed a method of correcting spherical aberration by adjusting a position of a collimator lens, which converges or diverges a light beam, along the optical axis so as to change the incident magnification of the light beam incident to the objective lens (for example, refer to JP-A-2005-327396, FIG. 2).
In addition, it is defined that the sign of the incident magnification is positive when divergent light is incident to the objective lens and the sign thereof is negative when convergent light is incident thereto.
In the optical disc apparatus, for example, as indicated by the characteristic curve QMS35 in FIG. 2, at the temperature of 35° C., when the incident magnification is set to 0, that is, when the light incident to the objective lens is parallel light, the 3rd-order spherical aberration amount is designed to become 0 mλrms.
On the other hand, at a temperature of 0° C., as indicated by the characteristic curve QMS0, the 3rd-order spherical aberration becomes about 140 mλrms in a state where the incident magnification is 0. However, it can be observed that it is possible to suppress the 3rd-order spherical aberration amount to approximately 0 mλrms in the case of convergent light by which the incident magnification of the objective lens is set to about −0.005.
As described above, it is possible to correct the 3rd-order spherical aberration depending on temperature change by adjusting the position of the collimator lens along the optical axis.